Photographic process for preparing a screen structure for a cathode-ray tube

ABSTRACT

A process for preparing a screen structure for a cathode-ray tube of the aperture-mask type comprising placing the panel assembly including the aperture mask on a lighthouse and unshuttering the light source, thereby projecting light through the mask incident on the panel. During the exposure and after a delay following unshuttering, preferably of at least 0.4 seconds, a portion of the light transmitted through the mask is continuously measured, and the light source is continuously adjusted to produce a total luminous flux on the film. In a preferred form of the novel process, the intensity level of the incident light is restored to a predetermined intensity value after the exposure.

PREPARING A SCREEN STRUCTURE FOR A CATHODE-RAY TUBE [72] Inventors: William Joseph Maddox; Morris Robert Weingarten, both of Lancaster, Pa.

[73'] Assignee: RCA Corporation [22] Filed: June 1, 1970 21 Appl. No.: 42,322

[52] U.S.Cl ..95/1 [51] Int. Cl. [58] Field of Search ..95/1

[56] References Cited UNITED STATES PATENTS 3,3l9,556 5/1967 Fiore ..95/1 3,395,628 8/1968 Kautz et al ..9 5/l POWER CONTROL UNIT CYCLE CONTROL 3 United States Patent 1151 3,636,836 Maddoxet al. 51 Jan. 25,1972

[54] PHOTOGRAPHIC PROCESS FOR 3,420,150 1/1969 Kaplan ..9s/1

Primary Examiner-Samuel S. Matthews Assistant Examiner-Richard M. Sheer Attorney-Glenn H. Bruestle ABSTRACT A process for preparing a screen structure for a cathode-ray tube of the aperture-mask type comprising placing the panel assembly including the aperture mask on a lighthouse and unshuttering the light source, thereby projecting light through the mask incident on the paneL- During the exposure and after a delay following unshuttering, preferably of at least 0.4 seconds, a portion of the light transmitted through the mask is continuously measured, and the light source is continuously adjusted to produce a total luminous flux on'the film. In a preferred form of the novel process, the intensity level of the incident light is restored to a predetermined intensity value after the exposure.

5 Claims, 8 Drawing Figures PATENTEU M25 1972 SHEEI 1 OF 4 CYCLE CONTROL INVENTORS Wzllzqn J. Maddox and Moms R. Wem

garten ATTORNEY mamwmesmz 3,636,836

SHEEI 2 0F 4 INVENTORS. William J. Maddox and Morrz's R. Weingarten.

ATTORNEY PAIa-ENTED JAHZS 1972 3:636 836 sum 3 0M I INVENTORS. WzlIz'arn J Maddox and Morris R. Weing'arten.

LAY

IIIIIJ 4 u ATTORNEY PATENTEDHANZSIQIZ 3.636336 SHEU t 0F 4 -EXPOSURE- (TOTAL LUMINOUS FLUX Fig. 7.

PATTERN 01 SIZE (M|LS) 12.5 12.? I29 m m 15.5 m

MASK HOLE SIZE (M|LS) Fig. 8.

- A INVENTORS. William J. Maddox and yorrz's R. Weingarten.

ATTORNEY PIIOTOGRAPIIIC PROCESS FOR PREPARING A SCREEN STRUCTURE FOR A CATIIODE-RAY TUBE BACKGROUND OF THE INVENTION This invention relates to a novel process. for preparing a screen structure for a cathode-ray tube. The novel process may be used to prepare, for example, a mosaic phosphor screenor a light-absorbing matrix for a color-television-picture tube.

' Color-television-picture tubes which include a mosaic phosphor screen and a light-absorbing matrix as structural parts of the luminescent screen have been described previously, for example, in US. Pat. No. 2,842,697 to F. J. Bingley and No. 3,146,368 to .I. P. Fiore et al. These patents describe color-television-picture tubes of the shadow-mask type in which a light-absorbing matrix is located on the inner surface of the faceplate of the tube. In this structure, the matrix has a multiplicity of holes therein, with a phosphor element filling each hole in the matrix.

The direct photographic method for preparing a matrix or a phosphor screen usually includes coating the inner surface of the faceplate of a cathode-ray tube with a film of photosensitive material composition (typically comprising a dichromatesensitized polyvinyl alcohol). A mask assembly is positioned in the faceplate and the entire panel assembly placed on a lighthouse. Multiple light beams projected through an aperture mask expose multiple patterns of the apertures in the mask on selected regions of the film. After exposure, the panel assembly is removed from the lighthouse, and the film is flushed with water to remove the still soluble regions while retaining the insoluble regions in place.

Ideally, the exposed patterns formed on the film by each of the multiple light beams are of uniform size and shape. In practice, the ideal is not achieved, resulting in an exposed pattern of a different size and shape from each beam. This is because the total luminous flux incident on the film is different for each of' the multiple exposures. Also, variations in mask transmission may produce variations in the size of the exposed areas of the film. Generally, the greater the mask transmission, the larger the size of the exposed area.

When the mark transmission is measured during the exposure, additional variations maybe introduced by transients andnonequilibrium conditions in the measuring system. Also, variables may be introduced when the exposure cycle is not duplicated in successive exposures.

SUMMARY OF THE INVENTION The novel method is used to prepare a screen structure for a cathode-ray tube having a panel assembly comprising a faceplate: panel including a viewing window and an aperture mask. The method includes coating a photosensitive composition, whose solubility is altered when it is exposed to light, upon the innersurface ofithe viewing window. Then, an aperture mask is positioned in the panel opposite the photosensitive coating, and the'panelassembly is placed on a lighthouse having'a normally shuttered light source that emits light continuously. The light source. is unshuttered, and light having a predetermined initial intensity level is projected upon the film for a predeterminedtime. During the exposure and after a delay following unshuttering, preferably, of at least 0.4 seconds, a portion of the light transmitted through the mask is continuously measured, and a signal is produced which is indicative'of the intensity of the light transmitted through the mask. The light source is then continuously adjusted in response to the'signal to produce a desired total luminous flux incident on the film.

Delaying measuring the light following unshuttering, preferably for at least 0.4 seconds assures that the light sensor is properly positioned and allows the system sufficient time to come to equilibrium.

In a preferred form'of the novel process for preparing asuccession of luminescent screen structures, the intensity of the incident light is'restored to the predeterminedinitial intensity value after each exposure is completed to provide for the same initial exposure conditions at the beginning of each cycle. In addition, restoring the light to the predetermined initial intensity results in longer lamp life.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 3 is a partial electrical schematic circuit diagram of the cycle control system of FIG. I.

FIG. 4 is a graph illustrating the time delay operation of the cycle control system of FIG. 3.

FIG. 5 is a schematic diagram illustrating a projected light beam exposing the photosensitive film with the apparatus shown in FIG. 1.

FIG. 6 shows a broken-away plan view of the film having a group of three exposed patterns obtained by three exposures by projected light beams in the manner shown in FIG. 3.

FIG. 7 is a graph including a curve illustrating the total luminous flux required to expose the film (exposed dot pattern size).

FIG. 8 is a graph including curves illustrating the variation of matrix pattern hole size with mask aperture size of the novel process and of-a prior art process.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is shown a faceplate assembly 10 comprising a faceplate panel 11, a photosensitive film 12 on the inner surface of the panel 11, and an aperture mask 13 positioned near and spaced from the photosensitive film 12. The faceplate assembly 10 is shown positionedon a photoexposure apparatus known inthe art as a lighthouse" 14. The lighthouse I4 is designed to expose the film 12 by projecting light from a small area light source incident on the mask 13 permitting beamlets l5of light to pass through the apertures 16 to forma pattern substantially of the same shape as the apertures 16 in the mask 13 on the film 12. The light for one such beamlet I5 is shown inFIG. l by the dashed line and schematically illustrated in FIG. 5 which will be describe later.

The lighthouse 14 comprises a housing 17 which includes a panel support 18, a light source 19, a reflector 20, a collimator 21, an eclipser 22 and a lens assembly 23. The panel support 18 is adapted to support the faceplate panel 11 accurately alignedover the light source 19 as shown'in FIG. 1. The lens assembly 23 includes correction lens 24, wedge lens 25 and a light filter 26 on the'upper surface of the wedge lens 25. The collimator 21 comprises alightpip'e in the form of atapered glass rod with the narrow end 27 constituting a small area source of light for the lighthouse 14. The eclipser 22 is operated by a solenoid valve 69to interrupt the light projected upon the photosensitivefilm l2. The collimator 21, eclipser 22, and lens assembly 23are positioned aligned over the light source l9 between the. light source l9and the faceplate assembly l0. Areflector 20 is positioned 'below the light source 19.

FIG. I includes a partial schematic block diagram of a lighthouse exposure control system. The control system comprises a light sensor assembly 28, a comparison unit 29, a signal condition unit 30, a power control unit 31 anda cycle control unit 70. The light sensor assembly 28 comprises a photosensor32 mounted in an enclosure 33. The enclosure 33 includes a threadedclamp 34'; a-retainer 35, and a flexible light shield 36.- The light sensor assembly 28 is adjustably mounted atone end ofanarm 37. The arm 37 is attached to a shaft 38 which ispivot'ally mounted-to the housing 17. A photosensor armactuator 67 operated by a solenoidval've 69 anda linkage (not'shown) pivot the shaft 38 to move the light sensor assembly 28 from a standby position to a position approximately centered over the faceplate assembly 10 as shown in FIG. 1.

in FIGS. 2 and 3, the light sensor assembly 28 includes a photosensor 32 which may be a photosensitive light-sensing cell. A preferred photosensor 32 is a photocell type 4403 sold by the RCA Corporation, Harrison, NJ. The comparison unit 29, a Wheatstone bridge circuit which is well known in the art, comprises the photosensor 32, two fixed resistors 39a and 39b, a standby resistor 40, a potentiometer 41, and an approximate 12-volt DC power supply 43. The preferred value for the two fixed resistors 39a and 39b is 12,000 ohms, and for the standby resistor 40, approximately 1,200 ohms. The preferred range of values for the potentiometer 41 is to 2,000 ohms. The signal conditioner unit 30 may comprise a direct-current amplifier 44 and an impedance matching circuit. A preferred amplifier is a transmitter model 2847-BX-20 sold by Acromag Corp., Detroit, Mich.

The power control unit 31 includes a controller 45, which may be of any type. A silicontrol SCR gate drive No. VS6734AF-1 l5 sold by Vectrol, lnc., Rockville, Md., is preferred in the actual operation of the power control unit 31.

Also included in the power control unit 31 are two siliconcontrolled rectifiers 46a and 46b, a ballast transformer 47, and a light source 19. The preferred rectifiers 46a and 46b are SCR type 2 N3899 sold by the RCA Corporation, Harrison, N.J.; the preferred ballast transformer 47 is transformer type 9T684322762 sold by General Electric Corp., Fort Wayne, 1nd.; and the preferred light source 19 is a lamp type BH-6 sold by the General Electric Corporation, Schenectady, NY. An AC power supply 48 which is approximately 180 volts (peak) is connected to the ballast transformer 47 through the rectifiers 46a and 46b.

The cycle-control unit 70 includes a cycle-initiate switch 63 connected to a first relay 65. A preferred first relay 65 is a relay catalog No. KAI lDG marketed by Potter Bromfield, Princeton, lnd. The first relay 65 includes first contacts 651: and second contacts 65b. A capacitor 64 is connected to the first contacts 65a. The capacitor 64 has a preferred capacitance value of 20 mfd. A diode 66, such as a silicon diode type lN1764 marketed by RCA Corp., Harrison, N..I., is connected through first contacts 65a of first relay 65 to provide a unidirectional, current flow from the AC power supply 48 to the capacitor 64. A timer 53, which includes first timer contacts 53a and second timer contacts 53b, is connected through the second contacts 65b of the first relay 65. A

preferred timer is a 0-15 minute timer, Cat. No. 305D0 l 5A20 PX, marketed by Automatic Time Control, King of Prussia, Pa. A solenoid valve 69 is connected to the second timer contacts 53b of the timer 53, and a second relay 42 is connected to the first timer contacts 53a of the timer 53. The second relay 42, which is a Relay Cat. No. 219ABAP marketed by Struthers Dunn, Pitman, NJ. includes first contacts 42a,

' second contacts 42b, and third contacts 420. The first contacts 42a of the second relay 42 are connected between the resistor 39b and the amplifier 44, the second contacts 42!) of the second relay 42 are connected between the photosensor 32 and the amplifier 44, and the third contacts 42c of the second relay 42 are connected across the Wheatstone bridge between resistor 39a and potentiometer 41, and resistor 39b and the photosensor 32.

In an automatic mode of operating the lighthouse of FIG. 1 for preparing a screen structure, a faceplate panel 11, having a photosensitive film 12 on the inner surface thereof and an aperture mask assembly 13 mounted therein, is placed in position on the panel support 18 as shown in FIG. 1. The lighthouse l4 operation is then initiated by actuating the cycle-initiate switch 63. Actuating the switch 63 closes the first contacts 65a and the second contacts 65b of the first relay 65. The closing of the first contacts 65a of the first relay 65 causes the capacitor 64, which is fully charged from the AC power supply 48, to discharge into the first relay 65, thereby maintaining the first contacts 65a'and the secondcontacts 65b of the first relay 65 closed until the voltage charge on the capacitor 64 has discharged to a level not sufficient to maintain the first relay 6S closed.

The closing of the second contacts 65b of the first relay 65 resets and initiates the operation of the cycle timer 53. Resetting the timer 53 closes the first timer contact 53a and the second timer contacts 53b. Closing of the second timer contacts 53!) operates the solenoid valve 69 to substantially simultaneously open the eclipser 22 and operate the photocell arm actuator 67 to pivot the light-sensor assembly 28 into the measuring position. Closing of the first timer contacts 530 connects the second relay 42 to the open position of the second contacts 65b of the first relay 65.

After the voltage level on the capacitor has decayed to a certain level, the contacts 65a and 65b of the first relay 65 open. Opening of the contacts 65a causes the capacitor 64 to recharge for the next cycle and causes the second relay 42 to operate. Operating the second relay 42 closes the first contacts 42a, the second contacts 42b, and opens the third contacts 42c. Closing of the contacts 421; and 42b, and opening the contacts 420 initiate the operation of the automatic exposure control system illustrated in FIG. 2.

When the eclipser 22 opens, light passes through the lens assembly 23 and the aperture mask 13, and falls incident on the photosensitive film 12. The predetermined initial light intensity on the photosensitive film 12 is preadjusted by a quiescent control input adjust 68 on the amplifier 44. With the third contact 420 closed, the DC power source 43 is connected to bias the amplifier 44 input to cause the light source 19 to be operated at the predetermined initial light intensity.

Since the second relay 42 is not operated until the capacitor 64 has discharged sufficiently to open the first relay 65, there is a delay before the exposure-control system operates. FIG. 4 illustrates a typical discharge curve 71 of the capacitor 64. The capacitor 64 discharges substantially exponentially from an initial supply voltage value (V,,) shown by the point 72 on the curve 71. At a lower voltage value (V 73, shown by the point 74 on the curve 71, the first relay 65 opens. The time between the time value at the initial voltage value 72 and the time value 75 at the lower voltage value point 74 is the amount of delay before the exposure system operates.

The time delay permits the light-sensor assembly 28 to be in position over the panel, eliminates the photosensor 32 from exposure to a no-light transient condition, and permits the photosensor 32 to stabilize at the predetermined initial light intensity as preset by the quiescent control-input adjust 68 on the amplifier 44. Therefore, when the automatic exposurecontrol system is activated, the photosensor 32 will be subjected to light at the predetermined initial light intensity. It is preferred that the predetermined initial light intensity be in the range of three quarters to full intensity of the average light intensity of the light source during operation.

In a preferred example, the delay can be calculated from the formula l/RC where V, is the supply voltage, V is the voltage value at which the first relay 65 will open, I is the time in seconds, R is the internal resistance of the first relay '65, and C is the capacitance value of the capacitor 64. The preferred values are R=10,000 ohms, V =l volts DC, V =20 volts DC, and 0 20 mfd. With these values, the delay time is approximately 0.44 second.

After the eclipser 22 is open, a portion of the light incident on the film 12 passes through the film 12 and the faceplate panel 11, and strikes the photosensor 32. The intensity of the light incident on the photosensor 32 produces an electrical signal from the photosensor 32. The signal from the photosensor 32 and a predetermined standard signal from the potentiometer 41 are compared in the Wheatstone bridge circuit, resulting in an error signal 49, which is a measure of the comparison. The error signal 49 is applied to the amplifier 44, where the signal is amplified and impedance matched, and the output of the amplifier 44 is applied to the controller 45. The controller 45'is of the type which continuously adjusts the insilicon-controlled rectifier 460 controls the power applied to the light source 19 during the positive half 51a of the cycle 50, and the silicon rectifier 46b regulates the power applied to the light source 19 during the negative half 51b of the cycle 50. The timing of each of the control pulses determines the portion of each half-cycle 51a or 51b during which the rectifiers 46a and 46b conduct, i.e., the portion of the sine wave during which power is applied to adjust the light intensity of the light source 19. If the timing of the control pulse is selected to occur at'some intermediate point such as 520 and 52b on each half-cycle 51a and 51b with a zero applied error signal 49, then a positive or negative error signal 49 will respectively move thetiming of the control pulse toward the beginning or toward the end of each half-cycle, increasing or decreasing the light intensity of the light source 19.

When the light intensity incident on the photosensor 32 is less than desired, a positive error signal causes the controller 45 to increase the power-level input to the light source 19, and when the light intensity incident on the photocell 32 is greater than desired, a negative error signal causes the controller 45 to'decrease the power-level input to the light source 19. For example, with a 6-millivolt error signal 49, an approximate 9 percent change is obtained in the light intensity of the light source 1 9. It should be understood that the light-exposure system provides for continual error signal power level control;

therefore, the light intensity of the light source 19 remains nearly constant within a predetermined range of values.

After a predetermined time, such as, for example, 8 minutes, the timer 53 shown schematically in FIG. 3 actuates a solenoid valve 69 to close the eclipser 22.

At approximately the same time, the second relay 42 operates to open the first contacts 42a and the second contacts 42b and to close the third contacts 42c. This causes the amplifier 44 to adjust the intensity of the light source 19 to the predetermined initial light intensity as determined by the input adjustment 68. When the lighthouse 14 is in the standby mode of operation, the light source 19 continually operates at the predetermined initial light intensity, until the next exposure cycle is initiated. Operation of the light source at the predetermined initial light intensity during the standby mode of operation also provides for optimum lamp life. After the exposure is complete, the light-sensor assembly 28 is pivoted to a standby position, and the faceplate assembly is removed from the lighthouse 14.

Several of the initial steps of the novel process for preparing light-absorbing matrix on the inner surface of a faceplate of an aperture-mask-type color-television-picture tube will now be described. First, the inner surface of a viewing window 11 of a cathode-ray tube is cleaned in the usual manner and then coated with a film of a photosensitive material whose solubility is altered when it is exposed to light, such as, for example, polyvinyl alcohol containing a soluble dichromate. After the film 12 is dried, an aperture mask is positioned in the panel in a predetermined position opposite and spaced from the film l2, and the assembly is placed on the lighthouse as shown in FIG. 1. The lighthouse in FIG. 1 has a normally shuttered light source that emits light continuously. In this example, the mask 13 has substantially circular apertures with a diameter of about 13 mils, and a center-to-center aperture spacing of about 28 mils near the center of the mask. The light source 19 is unshuttered, and light having a predetermined initial light intensity is projected through the mask assembly upon the coating for a predetermined time as previously described.

Then, after a delay of approximately 0.4 second, a portion of the light transmitted through the mask incident on the photosensor 32 is measured, and a signal 49 is produced that is indicative of the intensity of the transmitted light. The'intensity of the light source 19 is continuously adjusted in response to the signal 49 to produce a total luminous flux incident on the photosensitive coating.

After the predetermined time, the eclipser 22 is closed and the light source 19 is adjusted to operate at the predetermined initial light intensity as previously described. The panel-assembly is then removed from the lighthouse 14. Where a succession of panels is prepared one after the other, the process is repeated for the next panel. I

During the exposure, light from the lamp is projected through the lens assembly 23 and passed through the mask apertures 16 where beamlets 15 of light are incident upon the film l2 exposing a pattemon the film 12.

The exposure through the mask 13 is repeated three times, each time with the light rays incident at a slightly differentangle, so that the beamlets 15 of light provide patterns in groups of three, as in the usual method of shadow-mask-screen manufacture. Thus, at this point in the process, there are three exposed circular areas or dots each about 13 mils in diameter in the film for each aperture in the mask 13.

Following exposure, the assembly is removed from the lighthouse l4 and the mask 13 separated from the panel 11. The exposed coating is subjected to flushing with a forced spray of water for about 30 seconds, after which the panel 11 is rinsed with water and dried. At this point in the process, with the use of a positive photoresist, the faceplate surface carries an adherent stencil comprised of open areas and three patterns of dots of hardened polymeric film coated on the surface.

The stencil is now overcoated with a composition comprised of light-absorbing particles. In this example, the overcoating is produced by applying to the stencil a slurry containing about 5.0 weight percent of colloidal graphite in water and then drying the overcoating. It is desirable to include a trace of a wetting agent in the slurry in order to facilitate the spreading of the graphite slurry over the stencil. The overcoating is dried thoroughly for about 1.5 minutes with the aid of infrared heat. After cooling,,the overcoating is well adhered both to the polymeric dot patterns of the film and to the bare faceplate surface, which is the bare area of the stencil.

The overcoating is then wetted with water and drained. Next, before drying, a chemicallydigestive agent for the'film dot patterns is applied to the overcoating. In this example, the digestive agent is an aqueous solution containing about 6 weight percent hydrogen peroxide. This solution may be applied to the overcoating as a wash or as a spray under pressure. The hydrogen peroxide solution penetrates the overcoating and the film dot patterns causing the hardened polyvinyl alcohol of the film dot pattern to swell and soften. Subsequent flushing with water removes the softened film-dot pattern together with the overlying portions of the overcoating, but leaves behind that portion of the overcoating which is adhered directly to the surface'in the bare areas of the'screen structure. At this point, the faceplate carries a black, light-absorbing matrix having a multiplicity of circular holes therethrough about 16 mils in diameter.

The black, light-absorbing matrix is now rinsed with water and dried for about 4 minutes with the aid of infrared heat. Then, the faceplate is processed in the usual way to deposit red-emitting phosphor dots, green-emitting phosphor dots and blue-emitting phosphor dots overthe three patterns of holes in the matrix by the usual photographic techniques. The size and shape of the phosphor dots obtained are determined by the size of the film-dot pattern and will be discussed subsequently under the heading, Ph0toexposure." The completed screen has a matrix with three patterns of holes therein and phosphor dots substantially concentric therewith.

The luminescent screen may now be processed in the usual way to apply a reflective metal layer on top of the phosphor dots and the black matrix. The screen is baked and assembled with the aperture mask into a cathode-ray tube in the usual manner.

GENERAL CONSIDERATIONS The steps described above may be varied within limits and still fall within the scope of the invention. The process may be also applied to producing graphic images on other support surfaces, and for preparing other screen structures than that described. Some of the variations in the novel process are described below.

The Photosensitive Film The photosensitive film may be produced by coating a support surface, as by dipping, spraying, or flow coating, with a solution of polyvinyl alcohol containing a small amount of ammonium dichromate. The support may be the faceplate panel and may be a separate structure such as a glass plate.

Other photosensitive or photosensitizable materials may also be used. Some suitable water-soluble materials which can be made photosensitive are proteins such as gelatin, albumin and fish glue; carbohydrates such as gum arabic and starch; and synthetic materials such as polyvinyl pyrolhdone, and certain acrylic acid derivatives. In general, multifunctional watersoluble polymers containing reactive groups, such as OH, COOH, NH CO, singly or in combination, may be used. Mixtures of these materials may also be used. Some suitable solvent-soluble photosensitive materials are polyvinyl methyl ketone, KPR and KMER (available from Kodak, Rochester, N.Y.), aminated polystyrene, and hydroxy esters of polyacrylates. Water-soluble materials are preferred, at least because there is a large number of aqueous solutions that can be used in the subsequent step of graphic-image development. Solvent-soluble photosensitive materials are not as readily attacked by aqueous solutions. Suitable reagents for the graphic-image development of solvent-based photosensitive materials are acids, bases, and commercial strippers.

It is desirable that the photosensitive materials form a smooth, unbroken and uncrazed layer of film since this will produce the sharpest, cleanest graphic images. Therefore, it is preferred that the photobinder be film forming either directly upon deposition or during a heating step subsequent to deposition. In many systems, the film-forming temperature can be tailored by adjusting the relative proportions of ingredients constituting the photosensitive material. The film may also contain an auxiliary polymer to adjust the physical properties of the film. Where the photopolymer is principally dichromated polyvinyl alcohol, an acrylate polymer may be used for this purpose.

The Photographic Master Any pattern form may be used as a photographic stencil for exposing the photosensitive film. In preparing screen structures for color-television-picture tubes of the shadow-mask type by projection-type printing, it is preferred to use the aperture mask of the tube as a photographic stencil for exposing the photosensitive coating. In that case, the light source is placed at three separate locations in order to produce three separate exposures on the coating, each at a different location. Thus, three exposed patterns are produced in the graphic image for each aperture in the mask. Of course, the aperture mask or other stencil may be used to produce only one, or two or more exposures for the same coating. The pattern of the apertures in the aperture mask 13 determines the graphic image formed on the film 12. When a mask having circular apertures is used as a stencil, substantially circular dot patterns are formed on the film 12. The size of the dot patterns on the film 12 is determined by the total luminous flux incident on the film 12.

The Photoexposure The photosensitive material is exposed to a pattern of radiant energy in the range and of the type to which the photosensitive material is sensitive. When a preferred photosensitive material such as a solution of polyvinyl alcohol containing a small amount of ammonium dichromate is used, radiant energy in the form of ultraviolet light (about 3,650 A.) is satisfactory.

As previously discussed, the amount of light exposure determines the exposed pattern on the film. To expose equally the photosensitive film 12 for each pattern, the total luminous flux incident on the film for each pattern must be equal. The total luminous flux" is defined to be the summation of the incremental light intensities for all of the exposed area during the entire exposure time. To achieve this, either (a) the light source 19 is operated at a substantially constant light intensity and the time correspondingly altered as in a light-integrating control system, or (b) the time is maintained constant at a preset value and the intensity level of the light source 19 continuously adjusted within a predetermined range of values, as in an intensity control system. When three patterns are exposed on a particular faceplate in three separate lighthouses, it is preferred to use a predetermined time and to continuously adjust the intensity of the light source 19 to maintain the total luminous flux on the film 12 constant for each exposure. This permits a practical process since each lighthouse 14 completes its exposure cycle within the same time period.

FIG, 5 illustrates schematically the projected light as a beamlet 15 passing through one aperture in the mask shown in FIG. 1. Each beamlet 15 incident on the film 12 is comprised of two portions, a central beamlet portion 54 and a peripheral beamlet portion 55. The central beamlet portion .54 is comprised entirely of light rays falling from all portions of the light source 19 through the aperture 16 in a cylindrical form. The peripheral beamlet portion 55 is comprised of light rays from only a portion of the light source 19 falling through the aperture 16 in a conical form. The conical portion of the beamlet is sometimes called the penumbra.

FIG. 6 illustrates a portion of a group .of three separate dot patterns as is usual in the manufacture of shadow-mask tubes. Each pattern is obtained by three exposures from a beamlet 15 projected through the same mask aperture, shown schematically in FIG. 5, with each exposure made at a slightly different angle. The central beamlet portion 54 exposes a central dot portion 56 on the film while the peripheral beamlet portion 55 enlarges the dot by partially exposing a ring 57 around each central dot portion 56.

In FIG. 7, the curve illustrates the size of each pattern on the film, i.e., the diameter of the exposed dot pattern, is determined by the total luminous flux incident on the film 12. Viewing the curve shown in FIG. 7 it is observed that a minimum exposure time (minimum total luminous flux) is necessary to form an exposed pattern on the film, i.e., a pattern that is hardened and which will adhere to the support structure. This minimal amount of exposure is shown on the curve by point 58 and is defined to be the threshold of radiant energy necessary to produce an adherent pattern in the photosensitive film 12, since a photosensitive film such as ammonium dichromate sensitized polyvinyl alcohol does not adhere until a minimum amount of exposure is obtained.

After the threshold is reached, further exposure causes rapid dot pattern growth as approximately shown by the portion of the curve between first point 58 and second point 59. Further film pattern growth is obtained from the peripheral beam portion (penumbra) 55 as shown by the portion of the curve between second point 59 and third point 60. Since the light intensity is low in the penumbra 55, dot growth proceeds slowlyas shown by the slope of the curve between second point 59 and third point 60. After a maximum film size is reached as shown by the third point 60, further exposure produces substantially no additional dot growth.

Although a long exposure can be used to ensure full film pattern growth for each faceplate assembly, other problems such as excessive hardening of the pattern, fringe hardening from overlapping exposures, photoresist sensitivity variations, and processing time limitations make it preferable to stop the exposure before the third point 60 is reached. This requires the control of dot growth as by the novel process.

Although a 0.4-second delay after initiation of the lighthouse operation is described, this delay may be longer or shorter as shown in FIG. 4 by the curve 71. It is only necessary that the delay be sufficient to permit the photosensor to stabilize. It is preferred that the time delay be in the range of 0.4 to

Zseconds. The delay may also be obtained in a method other than described, such as, by the use of a time delay relay.

Pattern Size Control The exposure control system operates as previously discussed to control the total luminous flux incident on the photosensitive film 12. This permits printing of patterns substantially alike from each of the three exposures.

The control system also serves to provide an approximate uniform pattern size from a plurality of masks, each mask 13 having a different average aperture size near the center of the mask. Where the average aperture size in a mask 13 is larger than a particular size, decreased light is projected through the aperture,-and where the average aperture size in a mask is smaller than a particular size, increased light is projected through the aperture. Prior to the use of the novel process, for large size apertures, large pattern sizes resulted and for small size'apertures, small pattern sizes resulted. A more uniform pattern size regardless of minor variation in aperture size is obtained from the novel process.

FIG. 8 is' a graph including curves illustrating the variation of matrix film pattern size with the mask aperture size of the novel process and of a prior art process. The first curve 61 illustrates therelationship between aperture hole size and matrix hole size for the novel process, and the second curve 62 illustrates the same for a prior art process. The total luminous flux incident on the film is adjusted to expose an adherent l6.0-mil dot pattern for a l3.l-mil mask hole size. The slope of the first curve 61 of the novel process illustrates substantially less variation in pattern dot size over a range of aperture sizes than the second curve 62 for a prior art process. The novel process therefore provides substantially uniform screen structures for a succession of faceplates each having substantially different mask-aperture sizes.

Pattern Printdown This step is used only in the situation where it is desirable to produce the same pattern as the mask stencil, but that the pattern should be smaller in size than the stencil. in the case of making cathode-ray tubes-of the shadow mask type where the smaller pattern size results in a matrix tube known as a negative tolerance tube which is described in the prior art, it is desirable to have different pattern sizes than the stencil image, i.e the light-absorbing matrix (graphic image) has substantially the identical shape of holes but has a smaller pattern (hole) size than the holes in the mask. The novel process permits manufacture of matrix tubes of the negative tolerance type by repeatable control of the total luminous flux incident on the film 12 to obtain smaller patterns of a uniform size.

Referring again to FIG. 7, it can be observed that a pattern size smaller than the stencil occurs between first point 58 and second point 59; therefore, a pattern size smaller than the stencil can be obtained by actuating the eclipser 22 after a predetermined total luminous flux thereby stopping expbsure at a particular pattern dot size. Observing the slope of the curve between first point 58 and second point 59, it can be seen that the rate of dot growth is rapid; therefore control of total luminous flux incident on the film 12 is very critical to obtain. uniform dot patterns on the same film and on difierent films.

Stencil Development Where a photoresist technique has been used for producing the stencil, the exposed photosensitive film 12 is developed in the manner of the use for that material. In the caseof chromated polyvinyl alcohol, the development is carried out by flushing the surface of the film with water or with other suitable solvent for the unexposed, still-soluble photosensitive material. With other films, the same or other solvents may be used. The development should leave the minimum residue on the bared support surface so as not to interfere with the subsequent overcoating step.

overcoating The overcoating may be of any material which is adherent to the support surface. The overcoating may include a pigment or phosphor. Where it is desired to produce a light-absorbingmatrix for a cathode-ray tube, it is preferred to include in the overcoatinga relatively high loadingof-a dark pigment. The dark pigment is preferably elemental carbon in the form of carbon black, acetylene black, or graphite. Other dark pigments that may be used are silver sulfide, iron oxide, lead 'sulfide, and manganese dioxide. Generally, the pigment may be black, white or colored. Where itis desired to produce a luminescent structure, it is preferred to include a relatively high loading of phosphor particles in the overcoating. A process of this type for preparing patterns of. phosphor particles is disclosed in U.S. Pat. No. 2,840,470 toAL K. Levine. k

The overcoating must make a bond to the support surface that will endure the subsequent processing, such asremoving the image stencil and depositing the phosphor dots. With some materials, such as some commercially available dispersions of graphite in water, the graphite upon drying makes a bond to a glass faceplate which is adequate. With other materials, it may be necessary to include a small amount of abinder in theovercoating such that the dry overcoating developsa bond to the support surface through the use of the binder. Colloidal silica is a satisfactory binder for lamp black and acetylene black. For example, about 10 percentgof a colloidal silica with respect to the percent pigment present produces a strong bond to the glass faceplate especially where a small amount of ammonium dichromate is also present. Besides colloidal silica, alkali silicates may also be used as the binder.

Where a pigment is used for the purpose of making a lightabsorbing matrix for a picture tube of the shadow-mask type, the pigment must be deposited in sufficient density to develop the necessary opacity for this purpose. In the case of acetylene black and lamp black, the pigment should be deposited in a weight of about 0.2 to 2.0 mg./cm. of surface area and, preferably, about l.0 mg./cm. or more in order that sufficient thickness remains after tube processing. Where graphite or other pigments are used, slightly lower weights are required for achieving the same opacity in the final graphic image. I

The overcoating should also be penneable to and substantially unaffected by the graphic image developer, which must swell or erode or dissolve at least a part of the image stencil. Where the overcoating is entirely particles, it is necessarily permeable. Where the overcoating contains a binder, the overcoating may be permeable by nature or may be made permeable by crazing the overcoating. The bond between the supporting surface and the overcoating is preferably not substantially attacked by the graphic imagedeveloper. When the overcoating-support surface bond is both inert to the attack of the graphic image developer, and is adherent to the surface, it is possible to develop the graphic image after softening with a high-pressure spray of water, without any alteration of the pattern due to localized overdevelopment. If desired, appreciable amounts of organic material may be incorporated in the overcoating.

Graphic Image Development Any substance that dissolves or degrades the polymeric material of the stencil into soluble, partially soluble, or volatile fragments and leaves the oyercoating substantially unattacked may be used for developing the graphic image.

The preferred method for graphic image development is to apply to the overcoating'an aqueous solution of an oxidizing agent in a concentration stifficien'tly high such that rapid penetration of the overcoating and softeping of the stencil occur. in the case of stencils QfJi'ardened polyvinyl alcohol, the stencil softens rapidly with aqueous solutions containing between 1 and 35 weight percent hydrogen peroxide. Higher concentrations may also be used. Instead of hydrogen peroxide solutions, aqueous solutionsof the following may also be used: nitric acid, sodium peroxide, or both alkali peroxides, perchloric acid or alkali perchlorates, hydrofiorous acid, alkali hypochlorites, peracetic acid, alkali borates, alkali perborates, sodium hydroxides and certain enzymes. The graphic image developer solution is chosen so that it will not substantially decrease the adherence of the matrix overcoating to the substrate.

The time and temperature for carrying out the graphic image development are not critical, especially in view of the fact that they depend only on the removal of the polymeric material of the stencil. However, too fast a development may result in disruption of the overcoating,and too slow a development may result in the weakening of the bond between the overcoating and the support surface. Hence, in each case, the optimum time and temperature for image development are empirically determined.

Image development may also be carried out with nonaqueous reagents and mixtures of solvents and water-based reagents.

We claim:

1. In a method of making a luminescent-screen structure for a cathode-ray tube having a panel assembly comprising a faceplate panel including a viewing window and an aperture mask in a predetermined position spaced from said viewing window, said method comprising:

l. coating a photosensitive composition whose solubility is altered when it is exposed to light upon the inner surface of said viewing window,

2. positioning said aperture mask in said panel in said predetermined position opposite said photosensitive coating,

3. positioning said panel assembly on a lighthouse having a normally shuttered light source that emits light continuously,

4. unshuttering said light source and projecting light having a predetermined initial intensity through said mask assembly upon said coating for a predetermined time,

5. then, after a delay following unshuttering, continuously measuring a portion of the light transmitted through said mask and producing a signal which is indicative of the intensity level of said transmitted light,

6. and continuously adjusting said light source in response to said signal to provide a desired total luminous flux incident on said coating.

2. The method of claim 1 wherein said delay is approximately in the range of 0.4 to 2 seconds.

3. The method of claim 1 including the additional steps of:

7. moving a photosensor into position over said panel assembly simultaneously with unshuttering said light source,

8. producing a signal from said photosensor by intercepting a portion of the light transmitted through said viewing window,

9. and moving said photosensor away from said panel assembly.

4. In a method of making a succession of luminescentscreen structures for color-television-picture tubes, each tube of the type having a panel assembly comprising a faceplate panel including a viewing window and an aperture mask in a predetermined position spaced from said viewing window, said method comprising:

l. coating a photosensitive composition whose solubility is altered when it is exposed to light upon the inner surface of the viewing window in a first panel,

2. positioning an aperture mask in said first panel in said predetermined position opposite said photosensitive coating,

3. positioning said first panel assembly on a lighthouse having a normally shuttered light source that emits light continuously,

4. unshuttering said light source and projecting light having a predetermined initial light intensity level, through said mask assembly upon said coating for a predetermined time,

5. then, after a delay following unshuttering of at least 0.4 second, continuously measuring a portion of the light transmitted through said mask and producing a signal which is indicative of the intensity level of said transmitted light, I v 1 6. continuously ad usting said llght source in response to said signal to provide a desired total luminous flux incident on said coating,

7. shuttering said light source at the conclusion of said exposure time,

8. adjusting said light source to said initial light intensity level,

9. removing said panel assembly from said lighthouse,

l0. and repeating each of said foregoing steps for a second of said panel assemblies.

5. The method of claim 4 wherein said initial light level is within one half to three quarters of the average light level during said exposing step.

UNITED STATESPATEN'III O FICEQ CERTIFICATE 0F;C RR D Patent NO. 3,636,856 "pat-ea fi i Ir'wventloflsy William Joseph Madd x et I H v Q It is certified that error, appars i n'r ltheaboizvidefitifild and that said Letters Patent are her eby tzprire cted, as' shown below Column- 4; line 62 change; "V 3 I 0; r. .v 7

Column 9 lines 64 and 65 I t chan e d mafj i Signed zind sealed this 29th day of Aug l fl 97 '2 L (SEAL) Attest: I

EDWARD M.FLETCHER,'JR. I 5 ROBERT: GOTTSGHALKQ- Attesting Officert I Commissioner f-fiPattentg;

FORM PO-1050 (10 69) UNITED STATES PATENT OFFICE CERTIFICATE @E QQRREC'HQN Patent 3,636, 836 Dated January 25, 1972 IHVEMOMS) William Joseph Maddox et all It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 62 change "V 180'.- o.

Column 9, lines 64 and 65 change "chromated" to --dichromated- Signed and sealed this 29th day of August 1972 (SEAL) Attest:

LSD /IARD I' 'I.l*LETUHEI-L,JR. I ROBERT GOTTSGHALK Attesting Officer Commissioner of Patents )RM FO-IOSO (10-69) USCOMM-DC BO376-P69 Q U.S. GOVERNMENT PRINTING OFFICE: I959 0-366'334 

1. In a method of making a luminescent-screen structure for a cathode-ray tube having a panel assembly comprising a faceplate panel including a viewing window and an aperture mask in a predetermined position spaced from said viewing window, said method comprising:
 1. coating a photosensitive composition whose solubility is altered when it is exposed to light upon the inner surface of said viewing window,
 2. positioning said aperture mask in said panel in said predetermined position opposite said photosensitive coating,
 3. positioning said panel assembly on a lighthouse having a normally shuttered light source that emits light continuously,
 4. unshuttering said light source and projecting light having a predetermined initial intensity through said mask assembly upon said coating for a predetermined time,
 5. then, after a delay following unshuttering, continuously measuring a portion of the light transmitted through said mask and producing a signal which is indicative of the intensity level of said transmitted light,
 6. and continuously adjusting said light source in response to said signal to provide a desired total luminous flux incident on said coating.
 2. positioning said aperture mask in said panel in said predetermined position opposite said photosensitive coating,
 2. positioning an aperture mask in said first panel in said predetermined position opposite said photosensitive coating,
 2. The method of claim 1 wherein said delay is approximately in the range of 0.4 to 2 seconds.
 3. The method of claim 1 including the additional steps of:
 3. positioning said panel assembly on a lighthouse having a normally shuttered light source that emits light continuously,
 3. positioning said first panel assembly on a lighthouse having a normally shuttered light source that emits light continuously,
 4. unshuttering said light source and projecting light having a predetermined initial light intensity level, through said mask assembly upon said coating for a predetermined time,
 4. unshuttering said light source and projecting light having a predetermined initial intensity through said mask assembly upon said coating for a predetermined time,
 4. In a method of making a succession of luminescent screen structures for color-television-picture tubes, each tube of the type having a panel assembly comprising a faceplate panel including a viewing window and an aperture mask in a predetermined position spaced from said viewing window, said method comprising:
 5. The method of claim 4 wherein said initial light level is within one half to three quarters of the average light level during said exposing step.
 5. then, after a delay following unshuttering, continuously measuring a portion of the light transmitted through said mask and producing a signal which is indicative of the intensity level of said transmitted light,
 5. then, after a delay following unshuttering of at least 0.4 second, continuously measuring a portion of the light transmitted through said mask and producing a signal which is indicative of the intensity level of said transmitted light,
 6. continuouslY adjusting said light source in response to said signal to provide a desired total luminous flux incident on said coating,
 6. and continuously adjusting said light source in response to said signal to provide a desired total luminous flux incident on said coating.
 7. shuttering said light source at the conclusion of said exposure time,
 7. moving a photosensor into position over said panel assembly simultaneously with unshuttering said light source,
 8. producing a signal from said photosensor by intercepting a portion of the light transmitted through said viewing window,
 8. adjusting said light source to said initial light intensity level,
 9. and moving said photosensor away from said panel assembly.
 9. removing said panel assembly from said lighthouse,
 10. and repeating each of said foregoing steps for a second of said panel assemblies. 